Ganymede is unique among moons in that it has a magnetic field surrounding it, something even Mars does not possess. In December 1995, the Galileo spacecraft entered orbit around Jupiter. During a flyby of Ganymede at an altitude of only 838 kilometers, Galileo discovered a dipole magnetic field similar to the one surrounding Earth. The signature of Ganymede’s electric flux tube (the electric current that connects it with Jupiter) can be seen in Jupiter’s polar aurora.

With a mean diameter of 5262 kilometers, Ganymede is the largest moon orbiting any planet, and is the fourth largest rocky object after the planet Mars. Its magnetic field is supposedly created by the moon’s core in a “dynamo” of sorts—again like the Earth’s core is supposed to generate its magnetic field. There is an ambiguity, however: Ganymede’s core is thought to be too hot to hold on to permanent magnetism. On the other hand, Ganymede is so small that, according to conventional astrogeology, it should have cooled off billions of years ago and not have a liquid core in the first place.

The ad hoc explanation that NASA scientists announced also creates its own conundrum.

The moon once may have been much closer to Jupiter, so it was alternately compressed and stretched by the tidal forces of the planet’s gravitational field. The constant kneading of the moon kept its core liquid for much longer than if it had formed in its present orbit. If that were the case, then what forced an object bigger than the planet Mercury to move into a new orbit? Was any thought given to a mechanism for moving several quintillion tons of rock and ice a few thousand kilometers against the force of Jupiter’s gravity?

Of course, the most obvious aspect of Ganymede’s bizarre nature is its surface and the manifold examples of electric discharge machining (EDM). Previous Picture of the Day articles presented evidence for the hypothesis by showing how crater chains form on Ganymede and other planets and moons.

Another example of EDM is the huge circular structure dominating an entire hemisphere. Within the darkened circle, several bright craters are arrayed in a spiral. Directly below the circular depression are two other unusual craters, looking like the twin craters found on Mars. Some craters have rays extending outward for several kilometers in all directions. Several have one or more nested concentrically. Such features require a chain of unlikely coincidences if mechanical impacts are to explain them, but EDM creates such scars naturally

Could Ganymede’s magnetic field be related to the electrical phenomena that scarred and transmogrified it? If Ganymede was indeed closer to Jupiter at some point in its past, then wrenched from orbit and thrown thousands of kilometers farther out from the tidal grasp of its parent, could the force that was responsible for that event be electrical in nature? Was the moon gripped by an electrodynamic field large enough to imprint its core with permanent magnetism? What effect does the electrical connection with Jupiter have on Ganymede today?

ESA plans more missions to the moons of Jupiter in the next ten years. As more data is returned from a growing number of deep space probes, perhaps it will help to increase awareness for electricity in space.